What are you feeding her? As I recall, you said your puppy was a runt? Is she underweight compared to other puppies of her breed? Or is she actually underweight? If she is underweight compared to other puppies her age, she's supposed to be. She is a runt, after all. If she's eating regularly and has regular stools, this isn't really a health issue IMHO. She will probably end up being smaller than other dogs of her breed her whole life, feeding her to make her bigger will just make her fat, not healthy. If she is underweight because her ribs stick out overly far, you can see her hips and backbones, etc, and shes just scrawny/sickly, take her to a vet. Some runts can be much less healthy than normal sized dogs, this is why theyre referred to as runts.
3 things to look at. make sure you are feeding a Quality puppy food, not the walmart special. I use and prefere exclusive brand puppy, chicken and rice. it is not the only good one out there thoug. learn about foods and what you are feeding. Worms. it is very common for pups to have worms and that will suck up a bunch of the nutrients. if she has not been wormed in several weeks i would do it soon. Roundworms are the most common in this case. National brand dog food has a product called energy pack. you can order it from dresslers. It is 44% fat and somewhere in the 20's i think on protein. it is a SUPPLIMENT not a food. if you start her on it go slow, very gradually. for a dog like that you should not need it though. it is very hot stuff. and also remember puppies go through stages. sometimes they will eat a lot then for a while they slow down. sometimes they are growing out and sometimes up. it is not unusual for a pup to start to look thin during a big growth spurt. as long as she is getting the correct nutrients, a little on the thinner side is better then the fatter side anyway.
There is a recipe called Satin Balls which is excellent for putting weight on dogs or puppies. I have used it for one of my babies who was stricken with parvo and had lost a considerable amount of weight. I made the recipe actually cutting it in half and fed it to her 3 times a day and within a week she had gained all her weight back plus some. You have to be careful though or you will end up with an overweight dog. Here's the recipe it is also listed on the Holistic dog website which also contains lots of other great recipe's as well as remedies for common dog related health issues. http://www.holisticdog.org/Symptoms/symptoms.html SATIN BALLS 10 pounds hamburger meat [the cheapest kind] 1 lg. box of Total cereal [or comparable natural brand] 1 lg. box oatmeal 1 jar of wheat germ 1 1/4 cup veg oil 1 1/4 cup of unsulfured molasses 10 raw eggs 10 envelopes of unflavored gelatin pinch of salt Mix all ingredients together, much like you would a meatloaf. Divide into 10 quart freezer bags and freeze. Thaw as needed. Uses I take a frozen pack to dogs shows with me, kept on ice, and never have had any trouble with it spoiling, even in the heat of Texas summers... All my dogs go crazy over this stuff...but you have to be careful, it will put wt on the dogs pretty fast if you feed enough....I feed a pack a day...half in am , half in pm. When I want to increase the weight on a dog, I feed more; so far I haven't got to the point where they won't eat anymore, and believe me, Satin has eaten a lot at one setting. It will put on the weight, make the coats soft and blue black (on black dogs at least, that's all I have). The dogs stop all the itching and chewing at their coats/skin, their eyes get this bright look and the energy level goes out the roof(not that Belgians need any more energy). The bitch that I got this recipe for is a picky eater, but when this in on the food(I long ago stopped trying to "bury" it the other food; they just hunt it down to eat first, and making a mess trying) she eats much better. It seems to whet her appetite. Comments Per the information received after having the Satin Balls recipe checked by several vets/labs: Satin Balls is a total canine diet. It can be feed by itself or as a supplement, for however long you wish. My dogs have been on Satin Balls for over a year; the only time that I have fed it alone is when I had a sick dog needing to be built up or an underweight dog that I plan on showing. The only problem with feeding it by itself is figuring out the amount. It will put weight on a dog in a few days...that's why it is so great to feed just before a show. If you have a dog that is in good weight, but you just want to build coat/endurance, you would have to figure out how much to feed (cal per kg), or you would end up with a fat dog in a very short time. At one point, I let Satin eat as much as she wanted, just to see how much she would consume. I never got to that point! After a pound pack, she was still looking for more, so I stopped. I have been told a dog will stop eating when full on it, and that you can then gauge the amount needed to maintain weight!